That fits with my understanding of pubkey crypto basics, and of how quantum computers work. I think you need enough qbits to represent the binary digits of both prime numbers so that you can multiply them together and get them to match something which represents the message.

But I don't really know how the prime numbers I'm talking about map on to the keys and message cyphers used by PGP/GPG. Actually, I think the two primes relate to your public and private keys, but now I don't understand how they can be used to send a message such that the receiver can't decode your private key, so I'm clearly still missing something. And I haven't got the foggiest how you actually program a quantum computer, so it deffo sounds like I need to do some more learning.